How are standard php functions created?

Hello everyone. Recently a question arose: I would like to know if there are any resources, etc., where you can see how the standard php functions were created, well, that is, for example, the count function. How was it created, what elementary operators does it consist of?

Answer 1, authority 100%

go to here, download the sorts, then search them for any function you want.

Answer 2, authority 50%

PHP Source Code

Answer 3